1. Field of the Invention
The invention relates to a nozzle array for levitationally guiding and stabilizing web material for the purpose of contact-free heat transfer or drying, comprising a sequence of nozzle areas arranged in the transport direction of the web on at least one side of the web to be levitationally guided, the nozzle area including orifice nozzles or slotted nozzles.
Apparatuses for levitational guidance of web or strip material find many and varied applications in production technology. In textile production, webs of fabric are levitationally guided after printing. In drying plant, apparatuses for levitational strip guidance are employed downstream of painting systems in which both sides of the strip are painted or coated at the same time. In metalwork, levitational guidance of strips of sheet metal is employed in annealing when these need to be heat-treated on the fly without contact and with minimum stress.
2. Description of the Prior Art
Typical apparatuses of this kind are known from German disclosures DE-OS 25 56 442, DE-OS 30 26 132, DE-AS 14 74 239 and DE-OS 35 05 256. Common to all of these apparatuses is that nozzle ribs are arranged above and below the web transversely to the transport direction of the material which is guided horizontally as a rule. Located between these banks, by which levitational guidance is achieved by the corresponding flow forces, are the return flow surface areas for down flow of the gas blast jetted from the nozzle ribs against the web for levitational guidance thereof. In all of these apparatuses levitational guidance is accompanied by convective heat transfer for heating or cooling the levitationally guided web.
One such apparatus as known from German disclosure DE-OS 30 26 132, having a plurality of nozzle banks, is not only complicated in fabrication, but also has the functional disadvantage that the levitational force may be applied to the web only in the region of the projection of each nozzle rib and the space between the nozzle ribs needs to remain for the return flow. If the width to distribution ratio of the nozzle ribs is increased when a heavy web needs to be handled or in the case of the web needing to be supported at high temperatures and thus low gas density, then only a minor space remains available between the nozzle ribs for the return flow of the jetted gas. A likewise correspondingly larger proportion of the increase in pressure achieved by the fan circulating the gas in the apparatus is used up simply for the return flow.
This effect is particularly of disadvantage in an apparatus such as that disclosed by DE-OS 40 10 280 in which the return flow can only take place between the nozzle ribs. Increasing the flow rate jetted to the web which is to be levitationally guided fails namely to produce a corresponding increase in the supporting force, since at the same time the flow velocity needs to be increased in the restricted return flow cross-section, as a result of which the drop in pressure is increased in the region of the projection of return flow cross-sections at the web due to convective acceleration being likewise increased, i.e. although a larger overpressure is built up in the region of the nozzle ribs, at the same time, however, the vacuum pressure in the region of the return flow surface area is increased so that, in all, no substantial boost actually takes place.
The big disadvantage of existing apparatuses employing nozzle ribs and return flow surface areas between the nozzle banks for levitational guidance of wide webs becomes particularly evident when it is realized that for levitational guidance of a horizontal web an overpressure needs to be built up underneath the web, which on average corresponds to the unit weight of the web to be supported, i.e. a heavy web requires a correspondingly higher overpressure. Levitational guidance means that the web needs to be distanced away from the nozzle array located under the web, a volume thus materializing under the web which is formed by the surface area of the web and the distancing of the web from the nozzle array. The side surface areas of this volume are not restricted, i.e. the gas building up an overpressure under the web is able to flow from these side surface areas at a velocity corresponding to the overpressure relative to the environment. Thus, this lateral down flow will always materialize when the web is to be supported at a considerable distance away from the nozzle array. This considerable distance is, however, necessary when e.g. the material is a semi-finished strip of sheet-metal which deforms when exposed to the heat transfer occurring in levitational guidance, in this case spaces of 100 mm and more between the strip and nozzle array are needed. Achieving such a spacing in the case of heavy strips or webs is not possible with any of the apparatuses previously cited.